I. Field of the Invention
This invention relates to optical communication systems. More particularly, the invention is directed to lasers used in optical communication systems for providing electromagnetic energy at many different frequencies.
II. Background Art
High capacity optical communication systems require that many optical signals be frequency division multiplexed in the components of an optical network. This requires that there be a way of conveniently producing electromagnetic energy at many different frequencies. An ideal device for producing optical energy useful in an optical communication system is a laser. Recently, such lasers have been constructed as substrates or wafers comprised of semiconductor material having a frequency router connected to active sections, comprised of optical amplifiers, and selective sections, also comprised of optical amplifiers. A laser of this type is for example disclosed in commonly-owned U.S. patent application Ser. No. 08/019,952, U.S. Pat. No. 5,373,517, entitled Rapidly Tuneable Integrated Laser, filed Feb. 19, 1993, and is depicted in FIG. 1 of the drawings.
The laser 5 of FIG. 1 is seen to include a substrate or wafer 10 having a first cleaved face 20 and a second cleaved face 24, and formed in a manner well known to those of ordinary skill in the art. An N.times.N frequency routing device 12 is formed in the wafer 10 between the faces 20 and 24. One side of the laser 5--the left side in FIG. 1--is formed of a plurality of active optical amplifiers defining active sections 18.sub.1 through 18.sub.N, each in contact with a corresponding waveguide in a first plurality of wave 14.sub.1 through 14.sub.N. The waveguides are, in turn, connected to the left side of the frequency router 12. For lasing, one of the optical amplifiers 18 (such as the optical amplifier 18.sub.1 in FIG. 1) is DC biased in its active region so as to become light-transmissive and to provide gain to the selected lased frequencies, as more fully explained below.
The other side of laser 5--the right side in FIG. 1--has a plurality of selectively activatable optical amplifiers or gates 22.sub.1 through 22.sub.N which are placed at the second cleaved face 24 for forming a lasing path through which lasing occurs. Each selectively activatable optical amplifier 22 is connected to a corresponding one of the plurality of waveguides 16.sub.1 through 16.sub.N which are connected to the frequency router 12, i.e. to the left side of the router 12 in FIG. 1. The laser 5 is operated by forming lasing paths between one of the waveguides 14 which are connected to a corresponding active optical amplifier 18, and one or more of the waveguides 16 which are connected to the corresponding selectively activatable optical amplifiers 22.
Each optical amplifier comprises a doped section having controllable optical transmissivity. The doping level is such that an appropriately configured semiconductor junction is defined in each optical amplifier. These doped sections are optically active in that an application of electrical energy to those sections will cause them to become transmissive to the flow of optical energy and will even provide some degree of gain to optical signals flowing through them. When electrical bias current above a lasing threshold is applied, laser action begins. The doped sections are substantially opaque to the transmission of light when no electrical stimulation is applied. However, when electrical stimulation is applied, these sections become light transmissive. Thus, for example, a lasing path is created by applying the appropriate electrical stimulation, i.e. an amount above the lasing threshold, to optical amplifier 18 and to any of the selectively activatable optical amplifiers such as the amplifiers 22.sub.3 and 22.sub.6.
The frequency router 12 is a bidirectional device capable of multiplexing and demultiplexing optical signals and also possesses a wraparound feature. For an optical signal having a frequency F.sub.1 appearing on waveguide 14.sub.1 and flowing toward the router 12, the signal will be directed to the waveguide 16.sub.1. Conversely, an optical signal having a frequency F.sub.1 directed toward the router 12 on waveguide 16.sub.2 will be directed by the router to the waveguide 14.sub.1. An optical signal having a frequency F.sub.2 appearing on waveguide 14.sub.1 and flowing toward the router 12 will be directed to the waveguide 16.sub.2 and an optical signal having a frequency F.sub.2 directed toward the router 12 on waveguide 16.sub.2 will be directed toward guide 14.sub.1. Thus, by way of illustration, for a four port frequency router 12 having a signal F comprised of four discrete frequency components F.sub.1, F.sub.2, F.sub.3 and F.sub.4 appearing on waveguide 14.sub.1, the router 12 will demultiplex and direct the individual frequency components to waveguides 16 such that component F.sub.1 appears at waveguide 16.sub.1, component F.sub.2 appears at waveguide 16.sub.2, etc.
In addition and as stated above, the router 12 contains a wraparound feature which is inherent to the device. Thus, for a signal F having discrete frequency components F.sub.1, F.sub.2, F.sub.3 and F.sub.4 appearing on waveguide 14.sub.2 and flowing toward the router 12, the components will be directed to waveguides 16 such that frequency component F.sub.1 appears at waveguide 16.sub.2, frequency component F.sub.2 appears at waveguide 16.sub.3, frequency component F.sub.3 appears at waveguide 16.sub.4 and frequency component F.sub.4 appears at waveguide 16.sub.1. In other words, if the signal F appears at a waveguide 14.sub.2 which provides the signal to the router one waveguide position below the position of waveguide 14.sub.1, each frequency component appears at a correspondingly shifted waveguide. In this example, the described router 12 contains only four waveguides 16.sub.1 through 16.sub.4. Thus, frequency component F.sub.4 is wrapped around and routed to waveguide 16.sub.1.
When optical amplifier 22.sub.3 is activated by applying thereto an appropriate DC bias voltage, lasing at a frequency F.sub.3 occurs through the lasing path defined by waveguide 16.sub.3, frequency router 12 and waveguide 14. The laser 5 is capable of operating at two or more simultaneous frequencies and generates a multiplexed output made up of the lased signals. Thus, as seen in FIG. 1, gate 22.sub.6 is also activated which generates lasing at frequency F.sub.6 through a lasing path defined by waveguide 16.sub.6, frequency router 12, and waveguide 14.sub.1. The output signal accordingly contains frequencies F.sub.3 and F.sub.6. In operation, each frequency is modulated at a distinct rate (identified as M.sub.1 and M.sub.2 in FIG. 1 ), to provide data to the lased frequencies (F.sub.3 and F.sub.6), which data can be retrieved through known demodulation techniques.
While the prior art laser 5 of FIG. 1 is useful in optical communication systems because it is capable of operating at two or more simultaneous frequencies and generating a multiplexed output comprised of the multiple lased frequencies, the laser is nonetheless limited in several important respects. First, when such a laser is operated at two or more simultaneous frequencies, the output contains readily perceivable crosstalk. Second, because the modulation rate is related to the length of the lasing cavity, for the laser depicted in FIG. 1 the modulation rate is limited to about 150 Mb/s due to the relatively large size of the lasing path. This, of course, limits the amount of data that each frequency is capable of carrying. Thus, it would be desirable to have a digitally-tuned laser that is capable of operating at two or more simultaneous frequencies while eliminating crosstalk and appreciably increasing the attainable modulation rate.